The present invention relates to an electro-optic passive display cell comprising an active component imprisoned between two plates separated from each other by a frame to which the said plates are assembled tightly, the inner faces of the said plates carrying control electrodes and being covered with a coating of a material aligning the molecules of the active component.
Such cells are known per se. A solution for obtaining their active constituent consists in mixing dichroic molecules with a nematic liquid crystal having a positive dielectric anisotropy. These cells are easy to be realized and do not necessarily require polarizers, as it is generally the case for these display cells having a nematic helical liquid crystal. Unfortunately, these dichroic molecule containing cells effect a colorless display (in the activated zones) on a background the color of which is the complement of the color corresponding to the wave length or lengths absorbed by the dichroic molecules.
For removing this drawback, it has been suggested to use nematic liquid crystals having a negative dielectric anisotropy, the molecules of which are oriented homeotropically, perpendicularly to the plates, in the absence of an electric field. When an electric field is applied to the plates, the molecules become oriented in a direction parallel thereto. These type of cells need high electric fields since presently there do not exist nematic liquid crystals having a negative dielectric anisotropy the coefficient .vertline..DELTA..epsilon..vertline. of which is higher than 7, while, for the liquid crystals having a positive dielectric anisotropy, .DELTA..epsilon. can reach a typical value of 25. Moreover, one observer that without the presence of a polarizer, the contrast is generally insufficient.
It is known, especially as shown in the German Patent Application (DOS) No. 26 58 568, to use as an active component a mixture of a nematic liquid crystal having a negative dielectric anisotropy, dichroic molecules and a chiralic compound owing to which the mixture assumes a helical structure when an electric field is applied thereto. Such a cell effects the display as dark areas on a clear background. For the double purpose of increasing the commutation speed of the cell and of improving its contrast, the concentration of the chiralic compound is chosen in such a way that the pitch of the helix has a value of 0.7 to 10 times the value of the thickness of the cell.
Applicant has discovered that a cell as described in said German Patent Application (DOS) No. 26 58 568 fails to have satisfactory contrast for all range of concentrations indicated. The invention seeks to provide a display cell of this general type having improved performance and contrast characteristics.